Nanotechnology is a developing field with potential applications in a wide variety of industries. Currently, nanomaterials R&D is far outpacing research regarding potential health risks associated with exposure to these newly developed materials. Although there is no consensus definition for nanomaterials, several groups have defined nanomaterials as materials that are or contain features with at least one dimension in the nanoscale (100 nanometers or smaller).1 Recent literature has suggested that nanomaterials may have unique toxicological properties associated with their small size that may cause different or more potent toxicities than the traditional non-nano form. For example, NIOSH has established a separate recommended exposure limit (REL) for nano-titanium dioxide (TiO2) based on its unique toxicity when compared to the non-nano form.2 In addition to the nano size dimension, the unique shapes of some nanomaterials may also cause different toxicity than their non-nano counterparts. For instance, carbon nanotubes (CNT) have been proposed to cause a mesothelioma-like pathology upon interaction with human tissue that is not associated with other carbon particles.3

Despite the toxicity uncertainties, occupational health and safety professionals need tools to help them manage the potential risks that may be present in the workplace as a result of incorporation of nanotechnologies into industrial processes. The purpose of this two-part article is to provide OSH professionals the information necessary to evaluate the potential for worker exposure these materials and implement control strategies for mitigating unsafe conditions. In Part 1 of this series, we address methods of exposure assessment for nanomaterials in the workplace.

Exposure Assessment for Nanomaterials
The first step in protecting workers from unsafe exposure to nanomaterials is to understand whether and to what degree they are exposed. Assessing exposure to manufactured nanomaterials such as CNTs in an industrial facility presents special challenges because incidental nanoparticles formed from combustion processes and other indoor particulate create a mixed dust environment. Currently, there are no validated and/or substance-specific methods for assessing exposure to nanomaterials4, however, several guidance documents have been published and provide basic frameworks for developing an exposure assessment strategy.5-9 Strategies typically include the following basic steps:

Identification of materials of interest

Screening level assessment of exposure

Material-specific assessment of exposure

Evaluation of effectiveness of risk management measures, if instituted

Routine monitoring, where needed

Most exposure assessment methods are based on traditional industrial hygiene strategies, whereby the user performs a qualitative assessment first, followed by a quantitative or semi-quantitative assessment. For example, NIOSH’s Nanoparticle Emission Assessment Technique (NEAT) involves a tiered approach, which includes initial identification of possible sources of emission, conducting particle number concentration sampling to locate areas of suspected release, collecting filter-based samples at suspected release points, interpreting the data, and conducting routine or follow-up sampling.8-10 Additionally, Ramachandran et al. (2011) propose a strategy based on AIHA's exposure assessment strategy, which includes concentration mapping using continuous direct reading measurements in order to understand changes over time and space.6

While mass concentration (i.e., mg/m3) is used in many traditional industrial hygiene assessments, this metric may not fully characterize worker exposures because the nano particles do not have much mass yet could be present in relatively high concentrations by number count. Additionally, due to variations in biological responses to the nanomaterials, the metrics of interest may be material-specific. As such, it is important to obtain measurements of several metrics (e.g. number, mass, surface area) in order to understand the inter-relationship between metrics and to make comparisons.6 It also should be noted there are currently no legally enforceable occupational exposure limits (OELs) and very few recommended limits for nanoscale forms of most materials. However, given the potential differences in toxicity, care should be taken if using the OELs for the non-nano counterparts because they may not be sufficient for protecting workers.4-6, 11

Air Sampling Equipment
To conduct nanoparticle air sampling, a variety of instruments may be required because no single instrument is capable of meeting all needs of nanomaterials exposure assessment.

Particle counters are a popular choice for screening or mapping a facility due to their portability, relative low cost, and ease of use. In addition, particle counters can track changes in particle counts over time, which may provide insight about exposure during specific tasks or manufacturing activities. One of the most widely used nanoparticle counting methods is the paired use of an optical particle counter (OPC) and a condensation particle counter (CPC), which provide real time concentration ranges of particles by diameter.5 CPCs can count particles 10 to 1,000 nm in diameter, while OPCs count particles 300 to 10,000 nm.5 OPCs provide further breakdown of particle number concentration in multiple size channels and, when used together with a CPC, a semi-quantitative estimate of particles in the 10-300 nm size range is possible.4-6, 12

Other particle counting instruments include research-grade instruments, such as a scanning mobility particle sizer (SMPS) and the fast mobility particle sizer (FMPS). These instruments lack portability and are costly; however, they may be suitable for use in some environments.5 The SMPS displays number distributions of particles 2.5 to 1,000 nm in diameter in multiple size channels but takes two to three minutes to obtain a result, which makes it impractical for sampling in areas with variable aerosol levels.5 The FMPS takes less than one second to obtain a result; however, it has fewer channels than the SMPS.5

There are also direct-reading instruments available for measuring mass and surface area. For example, diffusion chargers are portable instruments with the capability of measuring surface area of nanomaterials in a size range of 100-1000 nm6, although they tend to underestimate the surface area of larger particles.5 Instruments such as an electric low pressure impactor (ELPI) and micro orifice uniform deposit impactor (MOUDI) provide real-time detection of number, mass, and surface area concentrations for particles as small as 6 nm; however, the high cost and low portability can make them impractical for use in many industrial environments.5,13 Other direct reading instruments, such as an epiphaniometer, can be used to determine surface area of aerosols less than 100 nm, but they are not suggested for use in manufacturing environments due to lack of a temporal resolution and use of a radioactive source.5,14 With regard to mass, the tapered element oscillating microbalance (TEOM) has a sampling inlet that can be used to select different size fractions. Photometers and other optical instruments are generally used only to obtain mass for particles greater than 100 nm and tend to have low counting efficiencies at smaller size ranges.5

While direct reading instruments provide general information about particles in the workplace, they are not capable of differentiating the particles of interest from other particulate in the air (background or process-related) by chemistry or morphology. However, filter-based samples can be used to learn specific information about particles of interest and to help differentiate exposure to ambient nanoparticles from those of the material of interest. Depending on the analyte, information from filter-based samples can include morphological features, surface area, chemical identification, mass, and other parameters.
NIOSH recommends taking a pair of samples: one to be analyzed for elemental mass (e.g., metals, elemental carbon, etc.) and one to be analyzed for particle characterization by electron microscopy.5 The choice of sampling method for chemical identification and/or microscopy will vary depending on several factors, such as the concentration of other contaminants in the environment, concentration of the material of interest, and limits of detection or quantification of methods available for analyzing the material of interest. As with traditional industrial hygiene methods, sampling times and flow rates will depend on the concentration of the analyte and other contaminants in the atmosphere, as well as considerations unique to the analytical method desired.5 In some instances, meeting mass-based detection limits for nanomaterials of interest (due to small mass) may be hindered by concentrations of other contaminants in the air.

While traditional industrial hygiene surveys rely heavily on personal sampling, a majority of devices designed for sampling nanoscale materials are made to collect area samples. In its NEAT strategy, NIOSH recommends that personal breathing zone samples be analyzed in the same manner as area samples (i.e., with electron microscopy and for elemental mass).5 The same challenges apply to the personal samples, such as adjusting loading to create suitable microscopy conditions or achieve low limits of detection for chemical analysis. Though not yet commercially available, the personal nanoparticle sampler (PENS), described by Tsai et al. (2012), collects both respirable particulate mass and nanoparticles using a respirable cyclone, micro-orifice impactor, and final filter.15

Conclusions
While the strategies for conducting exposure assessment for nanomaterials in industrial settings is similar to traditional industrial hygiene strategies, special considerations or equipment may be necessary in order to fully meet the exposure assessment needs. This field is ever evolving, and therefore it is important to keep current with the state of knowledge. One resource, the GoodNanoGuide, offers a repository for professionals to share their experiences in this field with others and to offer advice and guidance in the practical implementation of exposure and risk management strategies.16 This resource, along with emerging literature, can keep OSH professionals up to date on current efforts in this area.

Part 2 of this two-part article will be published in the September 2013 issue of OH&S.